The present invention relates to a coated cutting tool (cemented carbide insert) particularly useful for toughness demanding wet and dry machining, preferably milling, of low and medium alloyed steels and stainless steels, with raw surfaces such as cast skin, forged skin, hot or cold rolled skin or pre-machined surfaces under unstable conditions.
When machining low and medium alloyed steels and stainless steels with cemented carbide tools, the cutting edge is worn according to different wear mechanisms, such as chemical wear, abrasive wear, adhesive wear and by edge chipping caused by cracks formed along the cutting edge, the so called comb cracks. Under bad conditions, problems with bulk and edge line breakages occur commonly.
Different cutting conditions require different properties of the cutting insert. For example, when cutting in steels with raw surface zones or cutting under difficult conditions, a coated cemented carbide insert must consist of a tough carbide and have very good coating adhesion. When machining in low alloyed steels, and stainless steels the adhesive wear is generally the dominating wear type. Here, generally 1-3 .mu.m CVD- or PVD-coatings have to be used.
Measures can be taken to improve the cutting performance with respect to a specific wear type. However, very often such action will have a negative effect on other wear properties.
The influence of some possible measures is given below:
1.) Comb crack formation can be reduced by lowering the binder phase content. However, such action will lower the toughness properties of the cutting inserts which is not desirable.
2.) Improved abrasive wear can be obtained by increasing the coating thickness. However, thick coatings increase the risk of flaking and will lower the resistance to adhesive wear.
3.) Machining at high cutting speeds and at high cutting edge temperatures requires a cemented carbide with a rather high amount of cubic carbides (solid solution of WC--TiC--TaC--NbC). Such carbides will more easily develop comb cracks.
4.) Improved toughness can be obtained by increasing the Co-content. However, high Co-content will decrease the plastic deformation resistance.
So far it has been very difficult to improve all tool properties simultaneously. Commercial cemented carbide grades have therefore been optimized with respect to one or few of these wear types and hence to specific application areas.
Swedish patent application 9504304-8 which corresponds to U.S. Ser. No. 09/077,360 filed concurrently herewith (Attorney Docket No. 02444-499), discloses a coated cutting insert particularly useful for milling in low and medium alloyed steel with or without raw surface zones during wet or dry conditions. The insert is characterized by WC--Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an innermost layer of TiC.sub.x N.sub.y O.sub.z with columnar grains and a top layer of TiN and an inner layer of .kappa.-Al.sub.2 O.sub.3.
Swedish patent application 9501286-0 which corresponds to U.S. Ser. No. 08/616,012, herein incorporated by reference; discloses a coated cutting insert particularly useful for dry milling of grey cast iron. The insert is characterized by a straight WC--Co cemented carbide grade and a coating including a layer of TiC.sub.x N.sub.y O.sub.z with columnar grains and a top layer of fine grained textured .alpha.-Al.sub.2 O.sub.3.
Swedish patent application 9502640-7 which corresponds to U.S. Ser. No. 08/675,034, herein incorporated by reference, discloses a coated turning insert particularly useful for intermittent turning in low alloyed steel. The insert is characterized by a WC--Co cemented carbide body having a highly W-alloyed Co-binder phase and a coating including a layer of TiC.sub.x N.sub.y O.sub.z with columnar grains and a top layer of a fine grained, textured .alpha.-Al.sub.2 O.sub.3.
Swedish patent application 9503056-5 which corresponds to U.S. Ser. No. 08/703,965, herein incorporated by reference, discloses a coated turning cutting tool particularly useful for cutting in hot and cold forged low alloyed steel. The insert is characterized by a WC--Co cemented carbide body having a highly W-alloyed Co-binder phase and a coating including a layer of TiC.sub.x N.sub.y O.sub.z with columnar grains and a top layer of a fine-grained, .alpha.-Al.sub.2 O.sub.3.
Swedish patent application 9602413-8 which corresponds to U.S. Ser. No. 09/077,424 filed concurrently herewith (Attorney Docket No. 024444-497); discloses a coated turning insert particularly useful for turning in stainless steel. The insert is characterised by WC--Co-based cemented carbide substrate having a highly W-alloyed Co-binder phase and a coating including an innermost layer of TiC.sub.x N.sub.y O.sub.z with columnar grains and a top layer of TiN and an inner layer of fine grained .kappa.-Al.sub.2 O.sub.3.